Structural products with a closed polygonal section (e.g. rectangular box sections) or such as to present a high degree of rotational inertia (section irons) are widely used in various fields of industry for achieving a high degree of flexural and torsional rigidity.
Depending on the geometry of the finished product and the type and thickness of the material employed, such products are normally manufactured using conventional permanent deformation (pressing, bending) or extrusion techniques.
A drawback common to all the above techniques is the high cost of the tooling involved, which not only affects the price of the finished product, but also poses problems in terms of flexibility, by virtue of production machinery never, as a rule, being replaced until it is entirely written off.
In the case of permanent deformation techniques (pressing and bending), the severe stress required for achieving a high degree of local deformation (as when deep-drawing sharp edges, etc.) invariably results in springback of the material whereby, upon removal of stress, the pressed or bent part springs back to a different shape from that assumed under stress.
Dimensional inaccuracy resulting from springback of the material is especially noticeable when sheet metal parts produced using traditional permanent deformation techniques are welded, e.g. spot welded, to another part, in which case, any dimensional inaccuracy is counteracted by maintaining the two parts under stress and contacting each other. When stress is removed, however, the welded parts tend to return to their original shape, thus subjecting the weld spots to internal stress which remains throughout the working life of the assembled structure, and which, especially in the case of structures subjected repeatedly to pulsating stress (e.g. vehicles), results, at least macroscopically speaking, in sudden failure of the weld spots, which "spreads domino fashion" from one spot to another along the weld line, with catastrophic consequences to the structure as a whole.
The above defect, which is partly due to traces of oil in the liquefied material during welding, and to metallurgical irregularities and microcracks produced when the oil vaporizes, has partly been solved by pickling prior to welding. In addition to only partly solving the problem, however, pickling adds even further to the already high direct and indirect cost of conventional methods.
The above defect is also caused by electrolytic erosion resulting from moisture, water or dirt accumulating, with the aid of capillary attraction, in the weld spot region.
Extrusion processes also present several drawbacks, foremost of which are high cost, also in terms of energy consumption, and the invariably poor finish of the end product. Moreover, extrusion processes are limited to certain materials and to a very small range of shapes and sizes.
Traditional techniques for the manufacture of structural products thus leave ample scope for improvement.